Results and Discussion
The OD-Cu NSs were obtained by a facile microwave irradiation method with a reduction process. In a typical synthesis of Cu-30 and Cu-100, by adding 30 and 100 wt% of Cu(NO3)2∙3H2O to CuO NS, the OD-Cu NSs with a rough surface were formed in a large scale under the microwave irradiation with the power of 1050 W for 210 s. Figure 1a−d show TEM images of the pristine CuO NS (Figure 1a,b), as well as the microwave treated Cu-30 (Figure 1c) and Cu-100 (Figure 1d). The pristine CuO NS has an edge length of hundreds nanometer and smooth surface (Figure 1a,b). After the microwave irradiation, the OD-Cu NSs were obtained exclusively and maintained the original 2D morphology (Figure 1c,d). Interestingly, the nanosized domains (light grey circles in Figure 1c,d) were observed on the surface of Cu-30 and Cu-100 NS. The nuclei of Cu precursor salt could be randomly distributed on the nanosheet due to the polarity of ethylene glycol in a homogeneous mixture, then the copper precursor was rapidly reduced to metallic Cu under the microwave heating and formed the rough surface on the pristine CuO NS.30
To figure out their crystal structures, XRD measurements were conducted on the pristine CuO NS, Cu-30, and Cu-100 (Figure 1e). It is clearly shown that the pristine CuO NS consists of pure CuO phase (denoted as #). In comparison, a mixture of metallic Cu and Cu oxide diffraction patterns were observed in Cu-30 and Cu-100. Specifically, the peaks located at 43.3º and 50.4º predominantly in metallic Cu facets (denoted as *) are corresponding to Cu (111) and (200), respectively. The peaks at 35.5º and 38.7º assigned as CuO (002) and CuO (111) indicated the remains of CuO phase after the microwave process. Because CuO species have a higher dissociative energy than Cu precursors during the microwave-assisted reduction, the XRD patterns of Cu-100 show almost equal peak intensities of metallic Cu and CuO species, indicating that the ratio of oxide phase in Cu-100 was intensified by increasing the amount of Cu(NO3)2∙3H2O.31,32Beyond the dissociation energy, the insufficient reductant could further impede the reduction process of CuO NPs since ethylene glycol is rapidly consumed from the preferential reaction with NO3 and produced H2O at the initial stage.32 Additionally, the reductive rate between two distinct Cu species can be affected by the presence of additional Cu salt, resulting in the co-existence of Cu and Cu oxide phases in the OD-Cu NSs.
X-ray photoelectron spectroscopy (XPS) characterization was conducted to identify the surface valence states of Cu (Figure 2) and O (Figure S1). As shown in Figure 2a, all the samples disclosed three distinguished deconvolutions in the spectra of Cu 2p3/2, which can be assigned to Cu0/Cu+, Cu2+, and Cu(OH)2 species, respectively.33,34 It was notable that different Cu oxidation states co-existed on the surface of OD-Cu NS samples (Cu-30 and Cu-100). More importantly, the intensity ratio of Cu0/Cu+ to Cu2+for Cu-30 (1.48) is 8 times higher than that for Cu-100 (0.18), demonstrating that Cu-30 surface predominantly consists of Cu0/Cu+ rather than Cu2+ species. Furthermore, the Cu L3M4,5M4,5 Auger region around 568 eV was analyzed to clarify a specific chemical state of Cu element in Figure 2b, since the deconvolution of Cu0or Cu+ species is difficult to separate due to their similar binding energies.29,35 The peak position at 569.7 eV in Cu-30 sample corresponds to Cu+ species, whereas the Cu-100 exhibits a positive binding energy shift to Cu2+ state at 568.8 eV. These Auger XPS results demonstrate the surface of Cu-30 contains a high content of Cu+ species despite the absence of Cu2O signal in the Cu-30 XRD result. Considering previous reports revealed the relationship between the grain size and detection limitation for XRD inspection, the size of Cu2O grain was probably too small to be detected.36 Therefore, the targeted Cu oxidation state was achieved by simply adjusting the Cu salt amount under the microwave irradiation. Due to the surface oxidation from exposure to air, the commercial Cu NPs exhibit a chemical valence of Cu+ at 569.6 eV in Cu LMM peak (Figure 2b, black line). Moreover, as shown in Figure S1, the peak of oxygen species in the Cu2O lattice at 530.5 eV on Cu-30 is distinctly presented in comparison with that of Cu-100 in the O 1s XPS spectra.37-39
The electrochemical properties of each catalyst (commercial Cu NPs, CuO NSs, and OD-Cu NSs) were first examined by linear sweep voltammetry (LSV), which are shown in Figure 3a and S2. The LSV curves of these catalysts evidently show higher current densities in the CO2-saturated 0.1M KHCO3 than in the Ar-saturated solution, illustrating the benefits from the electrocatalytic activities for CO2RR. The catalytic CO2RR performance of the OD-Cu NSs was assessed in the potential window between −0.9 and −1.4 VRHE in a 0.1 M KHCO3 electrolyte. In comparison, the commercial Cu NPs and CuO NSs were also tested. In Figure 3b and S3, the overall FE was presented at each potential and ethylene was detected as the main product for each catalyst. Remarkably, the maximum FEC2H4 of 47% was obtained on Cu-30 sample at a low applied potential (both at −1.1 and −1.2 VRHE), which is obviously higher than those of Cu NP (26%), CuO NS (40%), and Cu-100 (40%) at the same potential, i.e., −1.1 VRHE (Figure 4c). Furthermore, as shown in Figure 4c, Cu-30 could reach the highest FE of 72% at −1.1 VRHE for total C2+products (C2H4, C2H5OH, C3H7OH, and acetate), while only 41%, 58% and 60% were achieved on the commercial Cu NPs, CuO NSs, and Cu-100, respectively. The current density was also improved by Cu-30 during the CO2RR (Figure 4e and S4). Specifically, the partial current density for C2+ products (jC2+ ) of Cu-30 achieved 29 mA cm−2 at −1.1 VRHE, which is 5 times higher than that of the pristine CuO NSs. The maximum jC2+ of Cu-30 and Cu-100 reached up to 55 and 52 mA cm−2 at −1.4 VRHE, respectively, which are higher than 42 mA cm−2catalyzed by the pristine CuO NSs, illustrating that the metallic Cu phase in OD-Cu produced with the microwave treatment could promote higher conductivity.
Stability is another crucial parameter to evaluate the catalytic performance. The FE on Cu-30 was monitored through continuous electrolysis for 8 hours at −1.1 VRHE. As shown in Figure S5, the FEC2H4 remained > 40% for 8 hours stability test while the sum of FEs for CO and CH4constantly kept less than 10% , suggesting that the production of low-valued products was effectively suppressed on the OD-Cu NSs surface. According to the aforementioned findings, the partially oxidized Cu species on the surface of Cu-30 could promote the *CO dimerization process, leading to the highest FE for ethylene.
To figure out the catalytic active sites and influential factors toward the improvement of FEC2+ on Cu-30 during CO2RR, the electrochemical surface area (ECSA) was calculated by cyclic voltammetry with different scan speeds under CO2-saturated 0.1 M KHCO3 solution after 1 hour catalysis in Figure 4a and S6. By calculating within the double layer charge region, the capacitance values of commercial Cu NPs, CuO NSs, Cu-30, and Cu-100 are 0.48, 1.4, 2.7, and 1.9 mF, respectively. Assuming the theoretical specific capacitance of polycrystalline Cu (pc-Cu) to 29 μF, the surface area of Cu-30 was significantly enhanced to 93 cm2ECSA, much higher than 16.5 cm2ECSA of commercial Cu NPs, 48 cm2ECSA of pristine CuO NSs and 65.5 cm2ECSA of Cu-100.40Although the surface morphologies of CuO NSs and Cu-30 are comparable, the ECSA of Cu-30 is twice as large as that of CuO NSs due to the unique partial oxidation state of Cu+ surface and the abundant defect sites provided by the nano-domains.19Accordingly, the enlarged nano-domains on Cu-30 were obtained after 1 hour electrolysis (Figure S7) and achieved the highest roughness factor of 93 based on defining pc-Cu as one.41
It is notable that the dissociation of CO2 molecules is one of the rate determining stages, also the energy barrier of CO2 decomposition (the absorbed dissociative CO2 → *CO intermediate + the absorbed O) is highly related to the surface oxidative state, especially Cu+ and intrinsic defective sites.42,43 In parallel, the crucial step for the formation of multicarbon products, C-C coupling, is associated with strong binding energy of *CO intermediates on Cu surface. Considering the dissociative CO2 is thermodynamically unstable in the electrolyte, the analysis of OHadsorption is a surrogate to inspect the binding energy of dissociative CO2 adsorption on Cu surface.42-46 A further investigation for OH adsorption was characterized by the electrochemical measurement in an Ar-saturated 0.1 M KOH electrolyte in the potential window of 0.3−0.6 VRHE. As shown in Figure 4b, except for the commercial Cu NPs, the Cu(100) peak (0.2−0.3 VRHE) and the Cu(111) shoulder peak around 0.48 VRHE were observed on the samples of CuO NSs, Cu-30, and Cu-100.43,45 The blue dash arrows obviously pointed out that the corresponding adsorption potential of Cu-30 negatively shifted compared with those of CuO NSs and Cu-100. This shift indicates a stronger adsorption energy with the dissociative CO2 on Cu-30 surface, which facilitated the C−C coupling process due to the immobilization of the dissociative CO2 intermediate.44,45,47,48 It is well matched with previous reports that the presence of partially oxidized Cu+ species on the surface generally lowers the activation energy to adsorb the dissociative CO2.42,49 Therefore, we conclude that the rough Cu surface formed on Cu-30 by the microwave treatment has great impact on the enlargement of surface area and the formation of the partially oxidized Cu species as active sites, promoting the C−C coupling process to enhance the CO2RR performances.